Now, a scientist named Stephan Gekle has found that you can make air move faster than the speed of sound by doing a simple little trick: throw a rock in a pond.

View video | As a disc representing a stone plunges into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in the video, the air is filled with smoke for visibility) shoots thro

Stephan Gekle/Physical Review Letters 2010

Gekle is a scientist at the University of Twente in the Netherlands who studies the physics of fluids. Physics is the study of forces and motion, and Geckle investigates how forces act on liquids, like water. In a recent study, he and his colleagues showed that after a rock drops into a body of water, a tiny jet of air shoots upward faster than the speed of sound.

This isn’t the first time Gekle has explored what happens when a rock sinks through water. In an earlier study, he and his team showed that as a rock falls into a flat surface of water, like a pond, it carves out a tiny tube of air. This tube connects the sinking rock to the air above the pond. The tube doesn’t exist for very long, though — almost immediately, the surrounding water pushes on the sides. This pressure is stronger in the middle than at the ends. As a result, the tube looks like an hourglass, where the middle gets smaller and smaller as the water forces the air out.

There’s not room in the hourglass for water and air, so as the water comes in the air escapes upward — and fast. These tiny jets of air can blast faster than the speed of sound, Gekle found.

To measure these air jets is trickier than it may seem. Gekle and his colleagues had to do more than stand at the edge of a pond with stopwatches. A careful science experiment requires a scientist to take multiple measurements of the exact same thing, to check and double-check the results. In this case, it would have been almost impossible for Gekle and his colleagues to throw a rock in a pond in the same way over and over again.

Instead, the scientists created a lab experiment that acted like a rock falling through water: They dragged a circular disc down through water at the same speed, over and over again, and watched what happened.

But there was another difficulty: It’s hard to see and measure air. To solve that problem, the scientists filled the air above the water with smoke and illuminated the smoke with a laser, which made the moving air easier to see. (To make the smoke, Gekle said, they used a smoke machine like the ones that provide the dramatic effects seen onstage at theaters.)

Finally, because everything happens so fast when the rock moves through water, the scientists had to find a way to slow down time. As the disc moved through the water, the scientists took pictures with a camera that captured 15,000 frames every second. (That’s faster than most movie cameras.) After the experiment, the researchers could slow down the movie and, aided by computer simulations, calculate the speed of air as it blew out of the hourglass-shaped tube.

But there’s one aspect of supersonic air that Gekle and his team didn’t observe. When a jet exceeds the speed of sound, the air around it produces a noise like thunder, called a sonic boom. So far, however, Gekle says the tiny air jets aren’t making even a teeny, tiny boom — but the researchers will keep listening.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

speed of sound About 760 miles per hour, through air at sea level.

supersonic Faster than the speed of sound.

physics The science of matter and energy and of interactions between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics and electromagnetism, as well as in modern fields including atomic and nuclear physics, solid-state physics, particle physics and plasma physics.

force The capacity to do work or cause physical change.

pressure Force applied uniformly over a surface, measured as force per unit of area.

Supersonic flows in action from Science News on Vimeo.

As a stone plows into still water, it plows out a column of air. The column collapses in an hourglass shape, and the escaping air (in this video, the air is filled with smoke for visibility) shoots through the shrinking opening at supersonic speeds.

Credit: Stephan Gekle/Physical Review Letters 2010

Going Deeper:

Fast-flying fungal spores from Science News on Vimeo.

Going Deeper:

Comet Tempel 1, as seen by the Deep Impact spacecraft as it traveled away from the icy body after the collision.

NASA, Deep Impact Team, University of Maryland

Some 80 telescopes on the ground and in space monitored and recorded the crash. Scientists have been analyzing these images, most of which feature the dust that blew off the comet for 2 days after the crash.

Initial observations suggest that scientists are for the first time “directly measuring pristine material from deep inside a comet, material that has been locked away since the beginnings of the solar system,” says Deep Impact researcher Carey Lisse. He’s at the University of Maryland and the Johns Hopkins Applied Physics Laboratory.

A haze of fine dust particles surrounds the impact site, as detected by cameras about a minute after the collision. Dredged up by a shock wave following the explosion, the dust lasted for hours and hid the crater gouged by the spacecraft’s projectile duri

NASA, Deep Impact Team, University of Maryland

Some of the results have been surprising. For one thing, scientists had long thought of comets as dirty snowballs—clumps of ice with bits of dust mixed in. Tempel 1, however, seems to be more of an icy dirtball.

The deep crater that formed after impact suggests that the comet is made up mostly of very fine dust, with some ice mixed in. Gravity holds the comet together only weakly, which makes it fragile and fluffy. The comet is so fluffy that about 80 percent its volume is empty space.

Nor does the comet consist of chunks of material squished together into one solid, as previously thought. Instead, it’s layered like an onion, say astronomers on the Deep Impact team.

A close-up view of Comet Tempel 1 just before a copper projectile plunged into its surface.

NASA, Deep Impact Team, University of Maryland

Figuring out how comets are put together should help astronomers understand how planets are created. Comets most likely formed 4.5 billion years ago, when the rest of the solar system was also born. Yet comets have experienced less heating and melting than planets and asteroids, so they hold insights about those early days.

Cameras captured great close-up pictures of the comet, and astronomers continue to marvel at Tempel 1′s many craters, which suggest a battered life. Images also show smooth surfaces and tall cliffs that scientists can’t yet explain.

As they continue with their measurements, calculations, and observations, astronomers hope eventually to get at the heart of what comets can tell us about the history of our solar system.—E. Sohn

Going Deeper:

Cowen, Ron. 2005. Deep impact. Science News 168(Sept. 10):168-170. Available at http://www.sciencenews.org/articles/20050910/bob9.asp .

Sohn, Emily. 2005. A smashing display. Science News for Kids (Aug. 24). Available at http://www.sciencenewsforkids.org/articles/20050824/Feature1.asp .

For further information about the Deep Impact space mission, go to www.nasa.gov/mission_pages/deepimpact/main/index.html (NASA) and deepimpact.umd.edu/home/index.html (University of Maryland).

The Hyper-X recently broke the record for air-breathing jet planes when it traveled at a hypersonic speed of seven times the speed of sound. That’s about 5,000 miles per hour. At this speed, you’d get around the world—flying along the equator—in less than 5 hours.

Powered by its scramjet engines (shown in gold), the black, unmanned X-43A flew at a record speed for an air-breathing jet plane. The experimental plane is only 12 feet long.

NASA

The Hyper-X is an unmanned, experimental aircraft just 12 feet long. It achieves hypersonic speed using a special sort of engine known as a scramjet. It may sound like something from a comic book, but engineers have been experimenting with scramjets since the 1960s.

For an engine to burn fuel and produce energy, it needs oxygen. A jet engine, like those on passenger airplanes, gets oxygen from the air. A rocket engine typically goes faster but has to carry its own supply of oxygen. A scramjet engine goes as fast as a rocket, but it doesn’t have to carry its own oxygen supply.

A scramjet’s special design allows it to extract oxygen from the air that flows through the engine. And it does so without letting the fast-moving air put out the combustion flames. However, a scramjet engine works properly only at speeds greater than five times the speed of sound.

A booster rocket carried the Hyper-X to an altitude of about 100,000 feet for its test flight. The aircraft’s record-beating flight lasted just 11 seconds.

In the future, engineers predict, airplanes equipped with scramjet engines could transport cargo quickly and cheaply to the brink of space. Hypersonic airliners could carry passengers anywhere in the world in just a few hours.

Out of the three experimental Hyper-X aircraft built for NASA, only one is now left. The agency has plans for another, 11-second hypersonic flight, this time at 10 times the speed of sound.

Hang on tight!—S. McDonagh

Going Deeper:

Weiss, Peter. 2004. Soaring at hyperspeed: Long-sought technology finally propels a plane. Science News 165(April 3):213-214. Available at http://www.sciencenews.org/articles/20040403/fob6.asp .

You can learn more about NASA’s Hyper-X plane at oea.larc.nasa.gov/PAIS/FS-2003-07-77-LaRC.html and www.nasa.gov/missions/research/x43-main.html (NASA).